Power amplifying apparatus and mobile communication terminal

ABSTRACT

A power amplifying apparatus includes an input terminal configured to receive an input signal, a first power amplifier biased for class A or class AB operation which is configured to amplify the input signal, an output terminal connected to an output of the first power amplifier, a second power amplifier biased for class C operation which is configured to receive and amplify a part of the input signal, and a switch connected between an output of the second power amplifier and the output terminal.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/840,691,filed Aug. 17, 2007, which is based upon and claims the benefit ofpriority from prior Japanese Patent Application JP 2006-238118 filed inthe Japanese Patent Office on Sep. 1, 2006, the entire contents of bothof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to power amplifying apparatuses. Inparticular, the present invention relates to a power amplifyingapparatus suitable for power amplification of OFDM (orthogonal frequencydivision multiplexing) modulated signals for a mobile communicationdevice and a mobile communication terminal which uses the poweramplifying apparatus.

2. Description of the Related Art

OFDM (orthogonal frequency division multiplexing) techniques have beendeveloped for increasing transmission rates of communication signalsused for mobile communication devices and improving frequencyutilization efficiency. OFDM realizes a very high transmission rate witha very narrow frequency bandwidth, making this technique attractiveespecially in an environment with limited frequency resources. However,a peak factor (a ratio of a peak power to an average power) in OFDM isvery large, such as 10 dB or larger, compared with that in othertechniques such as CDMA (code division multiple access) in which thepeak factor is approximately 3 dB. This large peak factor increases theamount of load imposed on power amplifiers for mobile communicationdevices or the like.

Power amplifiers for use in OFDM systems which are composed of singletransistors are known. Such power amplifiers can increase the outputlevel by no more than several dB even when distortion compensation isperformed, and thus may not be effective for a signal having a peakfactor as high as 10 dB. To address this shortcoming, a power amplifierin which separate amplifying elements each operating on the basis of anoutput power level and outputs of the amplifying elements are combinedhas been proposed. Examples of such a composite power amplifier includeLINC (linear amplification with nonlinear component) amplifiers andDoherty amplifiers. An amplifier configuration based on Dohertyamplifiers may be suitable to deal with the above shortcoming, takinginto account the characteristics of OFDM systems described above (seeJapanese Unexamined Patent Application Publication No. H7-022852).

SUMMARY OF THE INVENTION

In implementation of a Doherty amplifier, the performance is affected byimpedance variation at an output combining part that combines theoutputs of a so-called carrier amplifier and peak amplifier. A peakamplifier is configured to amplify a signal having a signal levelexceeding a predetermined threshold. When the peak amplifier is notoperating, the output impedance at the output combining part of theDoherty amplifier has to be open-circuited at high frequencies.

However, such an ideal state is difficult to achieve for the followingreasons. For example, when a field-effect transistor (FET) is used as apeak amplifier, its drain conductance is obtained from a carrieramplifier as a finite value. In general, this value exhibits largenon-linearity with respect to a drain voltage variation. Thus, radiationand distortion may occur in the peak amplifier when only the carrieramplifier is operating. In addition, the drain conductance is generallynot constant since it depends on the channel impedance of a device,which consequently causes design variation. When a hetero-junctionbipolar transistor (HBT) is used as a peak amplifier, the variation incollector conductance can be low compared with a FET. However, thenon-linearity of the collector conductance is large, and thus problemsassociated with distortion or the like still exist.

There has been no device that can realize the ideal state describedabove. Thus a technique which realizes an open-circuit-like state bycircuit arrangement has been developed. Such a technique is disclosedin, for example, Japanese Unexamined Patent Application Publication No.2005-117599. This technique is intended to reduce a loss of compositeoutput power which occurs when a back-off with which a power amplifieroperates is changed in accordance with the level of an input signal.However, this technique has a serious disadvantage in that theefficiency of an entire power amplifying apparatus is lowered whenchanging the back-off value.

Japanese Unexamined Patent Application Publication No. 2005-525727discloses another technique. In this technique, a plurality of auxiliarypower amplifiers are sequentially turned on in accordance with powerlevels so that the range of impedance change in power combining isincreased. This technique is intended to stabilize the performance of apower amplifying apparatus. However, with this technique, the number ofauxiliary amplifiers significantly increases, resulting in an increasein the size of the entire power amplifying apparatus and a decrease inefficiency.

The present invention has been made in view of the above circumstances.Accordingly, there is a need for a power amplifying apparatus that has asimple configuration and overcomes the problems of distortion andefficiency for a signal having a relatively large peak factor.

A power amplifying apparatus according to an embodiment of the presentinvention includes an input terminal configured to receive an inputsignal, a first power amplifier biased for class A or class AB operationwhich is configured to amplify the input signal, an output terminalconnected to an output of the first power amplifier, a second poweramplifier biased for class C operation which is configured to receiveand amplify a part of the input signal, and a switch connected betweenan output of the second power amplifier and the output terminal.

In this power amplifying apparatus, when the input signal has a lowpower level, only the first power amplifier operates and the secondpower amplifier is in a non-operating state. At this time, the switch isin an OFF state. Thus, even when radiation or distortion is generatedfrom the second power amplifier, the radiation or distortion is nottransmitted to the output terminal. When the power level of the inputsignal increases and exceeds a predetermined level, the second poweramplifier enters an operating state and the switch is turned on. Thus,the outputs of both the power amplifiers are combined.

This power amplifying apparatus is preferred for amplifying a signalhaving a relatively large peak factor such as an OFDM modulated signalused in a mobile communication terminal or the like.

In a power amplifying apparatus according to an embodiment of thepresent invention, even when radiation or distortion is generated from asecond power amplifier in class C operation during its OFF state, ahigh-efficient power amplifying operation can be performed on an OFDMmodulated signal without increasing distortion in a class A or class ABpower amplifier, by cutting off the second power amplifier from thepower combining point using a switch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram schematically illustrating a configuration ofa power amplifying apparatus according to an embodiment of the presentinvention;

FIG. 2 illustrates a first circuit configuration of a power amplifyingapparatus according to an embodiment of the present invention;

FIG. 3 illustrates an example of power variation of an OFDM signal withtime;

FIG. 4 illustrates a switching operation on an OFDM signal;

FIG. 5 illustrates a second circuit configuration of a power amplifyingapparatus according to an embodiment of the present invention; and

FIG. 6 illustrates an example of a modification of the second circuitconfiguration.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, the preferred embodiments of the present inventionwill be described with reference to the accompanying drawings.

Prior to the description of the preferred embodiments, a problemassociated with amplification of OFDM signals using a power amplifierwill be described. FIG. 3 illustrates an example of power variation ofan OFDM signal with time. One characteristic of an OFDM signal is a verylarge difference between the average power indicated as “mean” and peakpower indicated as “peak” as shown in FIG. 3. This difference may be 10dB or greater. The ratio of peak power to average power is called a peakfactor. In order to amplify a signal having a large peak factor using apower amplifier without producing distortion, a power amplifier having asaturation output power of 38 dBm or larger may be necessary where, forexample, an average output power of 28 dBm is desired. However, a poweramplifier in a mobile terminal generally operates at approximately 28dBm during a large part of transmission time, indicating a back-off of10 dB. In general, the efficiency of a power amplifier decreases withdecreasing output power. For example, in the case of a power amplifierhaving a saturation output power of 38 dBm, the efficiency at 28 dBmoutput power is as low as approximately 5 percent whereas the efficiencyat 38 dBm output power is 40 percent. This power loss may cause a mobilecommunication terminal serious problems in terms of continuous operationtime and heat output.

FIG. 1 is a block diagram schematically illustrating a configuration ofa power amplifying apparatus according to an embodiment of the presentinvention. This power amplifying apparatus is a composite poweramplifying apparatus having a first power amplifier 10 (PA_1) and asecond power amplifier 20 (PA_2).

The first power amplifier 10 is biased for class AB operation (class Abias may be applied instead of class AB bias). The second poweramplifier 20 receives a part of a signal SIGIN as input and is biasedfor class C operation. The outputs of both the power amplifiers 10 and20 are coupled via a switch 30 (SW). An output terminal of the poweramplifier 10 serves as an output terminal SIGOUT of the power amplifyingapparatus. When the input signal SIGIN has a low power level, the poweramplifier 10 is in an operating state and the power amplifier 20 is in anon-operating state. At this time the switch 30 is in an OFF state. Whenthe power level of the input signal SIGIN increases and exceeds apredetermined level, the power amplifier 20 enters the operating stateand the switch 30 is turned on, and thus the outputs of the poweramplifiers 10 and 20 are combined. The signals passing through the poweramplifier 10 and the power amplifier 20 are adjusted so as to have thesame phase value, so that the outputs of both the power amplifiers 10and 20 are combined while being in phase.

An operation of a power amplifying apparatus according to an embodimentof the present invention being applied to OFDM will be described. Theinput signal SIGIN illustrated in FIG. 3 is assumed to be an OFDMsignal. In this case, when the input signal SIGIN has a mean power value(mean state), only the power amplifier 10 operates. At this time, asillustrated in FIG. 4, the switch 30 is in the OFF state. The efficiencyof the power amplifying apparatus under this condition can be around 40percent. When the input signal SIGIN has a peak power value (peakstate), the power amplifier 20 operates and the switch 30 enters the ONstate. Thus, the outputs of power amplifiers 10 and 20 are combined inphase and the saturation output power increases.

Note that a class C power amplifier is generally capable ofhigh-efficiency operation and theoretically has an efficiency of 100percent (approximately 65 percent in practice due to signal degradationin a drive stage). Thus, a class C power amplifier can operate with anefficiency of approximately 26 percent while in the peak state. Theefficiency of the class C power amplifier in the peak state is lowerthan that in the mean state. However, influence of such a decrease inthe efficiency on a battery life and heating can be ignored since thetime period during which the power amplifier operates at the peak stateis very short.

FIG. 2 illustrates a first circuit configuration of a power amplifyingapparatus according to an embodiment of the present invention. In thispower amplifying apparatus, a part of an input signal SIGIN is dividedbetween a capacitor 31 (COP) and divided signals are input to the poweramplifier 10 and the power amplifier 20. A PIN diode 30 a (PIN_SW) isused as the switch 30 whose cathode terminal is connected to an outputterminal of the power amplifier 20 and whose anode terminal is connectedto the output terminal SIGOUT. The output terminal of the poweramplifier 10 is connected to the output terminal SIGOUT via a coil 32(L1).

Each of the power amplifier 10 and the power amplifier 20 is a FETamplifier having a two-stage dependent configuration.

The power amplifier 10 has FETs 12 and 16 and is class AB biased. Thegate terminal of the first-stage common-source FET 12 receives the inputsignal SIGIN through a matching circuit 11. The drain terminal receivesa drain voltage Vdd through a power supply coil 13. The drain terminalof the FET 12 is connected to the gate terminal of the second-stagecommon-source FET 16 through a matching circuit 14. The drain terminalof the FET 16 is supplied with a drain voltage Vdd through a powersupply coil 15 (Lab). The drain terminal of the FET 16 is connected tothe output terminal SIGOUT through a matching circuit 17 (M1) and a coil32 (L1). The matching circuit 17 has a coil 18 connected to the drainterminal of the FET 16 and a capacitor 19 connected between the coil 18and the ground. The matching circuit 17 optimizes constants to obtain amatching impedance ZL1 for maximum power output.

The power amplifier 20 has FETs 22 and 26, similarly to the poweramplifier 10. However, unlike the power amplifier 10, the poweramplifier 20 operates under a class C bias condition. The gate terminalof the first-stage common-source FET 22 receives the input signal SIGINthrough the capacitor 31 and a matching circuit 21. The drain terminalis supplied with a drain voltage Vdd through a power supply coil 23. Thedrain terminal of the FET 22 is connected to the gate terminal of thesecond-stage common-source FET 26 through a matching circuit 24. Thedrain terminal of the FET 26 is supplied with a drain voltage Vddthrough a power supply coil 25 (Lc). The drain terminal of the FET 26 isconnected to the output terminal SIGOUT through a matching circuit 27(M2) and a switch (PIN_SW) formed of the PIN diode 30 a described above.The matching circuit 27 has a coil 28 connected to the drain terminal ofthe FET 26 and a capacitor 29 connected between the coil 28 and theground. The matching circuit 27 optimizes constants to obtain a matchingimpedance ZL2 for maximum power output.

When the input signal SIGIN is an OFDM signal as illustrated in FIG. 3,only the power amplifier 10 operates and the power amplifier 20 is inthe non-operating state while the input signal SIGIN is in the meanstate. At this time, no current is flowing through the switch of a PINdiode 30 a and thus the PIN diode 30 a is OFF. When the input signalSIGIN is in the peak state, the power amplifier 20 is in the ON state,and a current flows in the drain terminal of the FET 26 through a coilLc. At the same time, a current also flows in the PIN diode 30 a througha path that includes Vdd, Lab, the coil 18 in M1, the PIN diode 30 a,the coil 28 in M2, and the drain of the FET 26 of the power amplifier20. Thus, the PIN diode 30 a is turned on. Operation performedthereafter is as described above.

According to the circuit configuration as illustrated in FIG. 2,switching timing of the switch 30 is autonomously determined. Thus, aspecific control circuit for the switch 30 is not necessary.

FIG. 5 illustrates a second circuit configuration of a power amplifyingapparatus according to an embodiment of the present invention.Components corresponding to those of the first circuit configurationillustrated in FIG. 2 are denoted by the same reference numerals, andthe description thereof will be omitted. In this embodiment, a FET 30 b(FET_SW) serves as the switch 30 and its source terminal is connected tothe output terminal SIGOUT. The drain terminal of the FET 30 b isconnected to the output terminal of the power amplifier 20 and the gateterminal of the FET 30 b is connected to a detector circuit 40 whichserves as a control signal generating circuit for generating a controlsignal (voltage) at the gate terminal of the FET 30 b for switchingoperation. This detector circuit 40 divides a part of the input signalSIGIN using a capacitor C1 and generates a detector output using adetector diode 45 (D1), a resistor 42 (R2), and a capacitor 41 (C2).This circuit extracts an envelope component and applies the envelope asa gate voltage of the FET 30 b. The cathode of the detector diode 45(D1) receives a reference voltage Vref through a resistor 43 (R1). Thisallows the setting of a power level (threshold) for ON/OFF operations ofthe FET 30 b.

An operation of a power amplifying apparatus having the aboveconfiguration when applied to an OFDM signal illustrated in FIG. 3 willbe described. As illustrated in FIG. 4, when the input signal is in themean state, only the power amplifier 10 operates and the power amplifier20 is in the non-operating state. At this time, the output level of thedetector circuit 40 is low and thus the FET 30 b is not turned on. Whenthe input signal SIGIN is in the peak state and the power amplifier 20is turned on, the output level of the detector circuit 40 is high andthus the FET 30 b is turned on.

In the circuit configuration illustrated in FIG. 5, there may be a delayin generation of a control signal by the detector circuit 40. This delaymay cause an ON/OFF timing of the FET 30 b to be shifted from a peak ofthe input signal SIGIN. To avoid this fault, a delay circuit 52 whichdelays the input signal SIGIN by a predetermined time is inserted in theupstream of the power amplifier 10. The delay circuit 52 can be formedof a combination of a resistor and a capacitor, for example, forproducing a delay time in accordance with a delay of a control signal inthe detector circuit 40.

While the preferred embodiments of the present invention have beendescribed above, various modifications and changes may be made to theembodiments. For example, in the foregoing, the case is described wherea FET is used as an amplifying transistor. However, it is also possibleto employ a bipolar transistor. When a bipolar transistor is used as thepower amplifier 20, the OFF state of the power amplifier 20 can beobtained with increased reliability.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factors insofar as they arewithin the scope of the appended claims or the equivalents thereof.

1. A power amplifying apparatus comprising: an input terminal configuredto receive an input signal; a first power amplifier configured toamplify the input signal, the first power amplifier being biased forclass A or class AB operation; an output terminal connected to an outputof the first power amplifier; a second power amplifier configured toreceive and amplify a part of the input signal, the second poweramplifier being biased for class C operation; and a switch connectedbetween an output of the second power amplifier and the output terminal,the switch autonomously controlling the output of the power amplifyingapparatus between an output of the first amplifier and a combination ofthe output of the first amplifier and an output of the second amplifier,wherein the switch controls the output of the power amplifying apparatusto be the output of the first amplifier when the input signal is aOrthogonal Frequency Division Multiplexed (OFDM) signal, and the switchcontrols the output of the power amplifying apparatus to be thecombination of the output of the first amplifier and the output of thesecond amplifier when the input signal is in a peak state.
 2. A mobilecommunication terminal provided with a power amplifying apparatusperforming power amplification on an OFDM modulated signal, the poweramplifying apparatus comprising: an input terminal configured to receivean input signal; a first power amplifier configured to amplify the inputsignal, the first power amplifier being biased for class A or class ABoperation; an output terminal connected to an output of the first poweramplifier; a second power amplifier configured to receive and amplify apart of the input signal, the second power amplifier being biased forclass C operation; and a switch connected between an output of thesecond power amplifier and the output terminal, the switch autonomouslycontrolling the output of the power amplifying apparatus between anoutput of the first amplifier and a combination of the output of thefirst amplifier and an output of the second amplifier, wherein theswitch controls the output of the power amplifying apparatus to be theoutput of the first amplifier when the input signal is a OrthogonalFrequency Division Multiplexed (OFDM) signal, and the switch controlsthe output of the power amplifying apparatus to be the combination ofthe output of the first amplifier and the output of the second amplifierwhen the input signal is in a peak state.